The present invention is directed to a set screw, especially a set screw for use in medical implants, that includes a step-down internal bore for improved interface with an easy-out for purposes of removal and an external thread that draw walls of the structure within which the set screw is placed toward the set screw rather than urging the walls away from the set screw.
Set screws are used in many ways in order to lock one element of a device relative to another. Set screws are quite important in the art of medical implants, as it is often necessary to capture one element of the implant relative to another and to then lock the two relative to one another to prevent subsequent movement therebetween. Failure to properly lock two elements of a medical implant together may result in failure of the implant and possible serious injury to the patient within which the implant is placed.
With medical implants, it is desirable to have very lightweight and low profile elements so that the overall implant impacts as little as possible upon the patient. If set screws could be manufactured that were quite large and the other elements of the implants could be likewise quite large and such would still have a low profile, it would be much easier to construct a suitable set screw in such implants. However, large size and low profile are incompatible goals. Size, weight and profile must all be taken into consideration and minimized, as much as possible.
In order to provide sufficient strength and friction to resist movement of the various elements, once the set screw is seated, it is necessary to apply a fairly substantial torque to the set screw. While some set screws are torqued without a head, many of the set screws currently in use in medical implants have a driving head that breaks away from the remainder of the set screw at a preselected torque in order to assure that the set screw is sufficiently torqued to provide the necessary strength and locking friction. The head is also broken away in order to assure that the set screw is not over-torqued and the threads stripped. Further, the head is typically broken away in order to provide the low profile and light weight that is desired in such set screws.
Because the driving head is typically broken away and because it is sometimes necessary to remove the set screw after implantation and setting thereof, some mechanism must be provided in order to remove the set screw. Various structures have been provided for this purpose in prior art devices. The prior art structures have had varying degrees of success, but have typically been most effective in set screws having a diameter that is comparatively large, such as 8 to 10 millimeters, because such larger set screws provide greater surface and volume to allow the placement of removal structure of one kind or another on the set screw.
Easy-outs are a commonly used tool that have been utilized to remove bolts and screws that have been used in various mechanical devices and that have no other means for gripping. Such have especially been used for bolts where the heads have been broken away. However, the term xe2x80x9ceasy-outxe2x80x9d is somewhat of a misnomer in that such are actually very difficult tools to utilize. This is especially true when dealing with set screws of the size used in medical implants which often range from 5 to 10 millimeters in diameter. It has been found that set screws of this size with a conventional axial bore are often not removable by an easy-out, because the easy-out has too little edge or surface upon which to grip. Further, the edge that has been previously provided is often torn away by use of the easy-out to a point where there becomes less and less edge to grip with each subsequent attempt. Consequently, it is desirable to produce a set screw having a head that breaks away from a base of the set screw at a preselected torque yet provides a highly gripable surface or edge on the set screw for use in conjunction with an easy-out design.
Another inherent problem in certain medical implants with set screws of a conventional type is that such set screws typically utilize threads which are referred to as V-threads. The edges of a cross-section of V-threads has a V shape. V-threads work reasonably well in devices where a bore is provided that completely surrounds the set screw and has a mating thread that mates with the thread of the set screw. However, many medical implants, such as open headed bone screws, do not provide for a bore that will entirely encircle the set screw. In such implants, the set screw also functions as a closure and spans between a pair of discontinuous threaded surfaces. When V-thread set screws are utilized for this purpose, the forces exerted by the set screw during torquing are partially parallel to the axis of rotation of the set screw and partially radially extending outwardly from the set screw. The radial outward forces can and frequently do spread the arms of the head within which the set screw is being torqued sufficiently to allow for failure of the set screw. Buttress-type threads have been utilized for the purpose of trying to reduce the radial outward forces that are exerted by the threads. In buttress-type thread screws, the trailing surface of the thread normally has a cross-section edge that is parallel to or is fairly close to being parallel to a radius of the set screw. Sometimes such surfaces are referred to as flat, but normally the cross section has a slight inclination of from 5 to 10 degrees so that a smaller, but yet substantial force, is exerted radially outward by the buttress thread screws as compared to the V-shaped thread screws. Consequently, it is desirable to also have a set screw of this type wherein the threads are designed to exert an inwardly directed force to pull opposing walls of an implant toward the set screw, rather than urge the walls away from the set screw.
A set screw having a threaded base and a driving head that is breakable from the base at a preselected torque at a breakaway region. A bore extends axially through the head and into the base. The bore has a first larger diameter through the head and a portion of the base and a second smaller diameter through a portion of the base providing a stepdown transition between the bores. The stepdown has an internal circumferential edge which cooperates with an edge of the larger diameter portion of the bore when the head is broken from the base, so as to provide two edges in spaced relationship to one another. In particular, the two circumferential edges are sized, spaced and positioned so as to mate with the face of an easy-out positioned therein. The angle formed by joining points on the edges together that are associated with a plane passing through an axis of rotation of the screw and the edges, is the same as the angle formed by an engaging face of the easy-out and an axis of rotation of the easy-out. In this manner the easy-out with a reverse threaded surface is able to simultaneously cut into and engage both of the edges. The force that is able to be exerted on the set screw through the easy-out is highly proportional to the length of the edges and/or surface that the easy-out can engage. Consequently, the two edges are designed and positioned to mate with the conical surface of an easy-out to allow for the best gripping of the easy-out relative to the base.
The base also includes an external helically wound thread. The thread has a leading surface and a trailing surface relative to advancement of the set screw when rotated about a central axis clockwise. A cross-section of the set screw in a plane passing through the axis of rotation forms intersections with the leading and trailing edges that both slope rearwardly from an inner edge to outer edge thereof.
Therefore, the objects of the present invention are to provide a set screw having a base and a head that breaks away from the base at a breakaway region, so as to provide a low profile subsequent to setting of the set screw; to provide such a set screw having an axially extending bore that passes through the head and has a stepdown region in the base wherein the bore passes from a region of a larger diameter to a region of a smaller diameter; to provide such a set screw wherein the breakaway of the head from the base produces a circumferential ring at the mouth of the bore; to provide such a set screw wherein the edge at the mouth of the bore and the edge at the stepdown region of the bore are sized, shaped and spaced so as to mate with a reverse threaded conical surface of an easy-out, so as to improve the engagement and gripping of the easy-out relative to the set screw base; to provide such a set screw having two or more step-down edges to improve gripping by an easy out; to provide such a set screw wherein the base has an external thread that has both a leading edge and a trailing edge that slope rearwardly with respect to clockwise advancement of the set screw in a bore from an inner diameter of the thread to an outer diameter thereof; to provide such a set screw that exerts both axial reactive forces and inward radial reactive forces on an implant bore within which the set screw is placed; to provide such a set screw that is highly effective at setting one element of an implant relative to another element of the implant, but is readily removed should removal be necessary; and to provide such a set screw which is easy to use and especially well adapted for the intended purpose thereof.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.